1. Field of the Invention
This invention relates generally to lithographic processing. More particularly, this invention relates to a system and method for monitoring the topography of a wafer surface during lithographic processing.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the relevant art would recognize that other substrates could be used without departing from the scope of the instant invention.
During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum or electrostatic chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized.
While using a step-and-scan technique generally assists in improving overall image sharpness, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, and the particular reticle being used. Such image distortions are frequently due to the poor focus that results from the wafer surface being located somewhere other than in the desired focal plane of the projection optics. Since the surfaces of wafers are seldom planar, especially after multiple processing steps, focus problems are often related to the inability to know precisely how far the wafer surface is from the projection optics along the illumination axis of the lithography apparatus. This, in turn, often stems from the fact that most typical sensors or gauges used to measure the separation between the projection optics and the wafer surface cannot be located along the axis of illumination.
What is needed is a system and method that can be used to determine the separation between a wafer surface and the projection optics along the axis of illumination so that accurate focus can be maintained.